The hybrid ring circuit, the so-called rat race, has been used for many years and is still an essential part of many complex microwave circuits such as mixers, filters, phase shifters and power dividers. The rat race has three of the four transmission lines between ports equal to one-quarter wavelength and one line equal to three-quarter wavelengths at midband. The isolation between any two opposite ports is infinite at midband because the two lengths differ by exactly by 180.degree. but drops rapidly with a change in frequency due to the change in relative path lengths. The useful bandwidth is about 10%. A modified ring circuit is described in "A Wide Band Hybrid Ring for UHF" by W. V. Tyminski, Proceedings of IRE, January 1953, p. 81-87 and is useful over more than an octave bandwidth. The modified hybrid ring described therein has four transmission lines between output ports each equal to one-quarter wavelength at midband and provides an even 3 db power split. The device, however, was originally used only in the low UHF band because it could not be fabricated for use at higher frequencies. A balanced-line hybrid for extending the operation into the microwave region was described by J. W. Carr in the Microwave Journal, May 1973, p. 49-52.
A microstrip hybrid directional coupler has been disclosed in which the 180.degree. phase shift is brought about by a twisted pair of parallel conductors. See U.S. Pat. No.4,023,123 issued May 10, 1977 to John Reindel.